Nanostructured alkali cation incorporated δ-MnO₂ cathode materials for aqueous sodium-ion batteries

Nanostructured δ-MnO₂ incorporated with alkali cations (A-δ-MnO₂, A = K⁺, Na⁺) has been synthesized and evaluated as a cathode material for aqueous sodium-ion batteries. It is observed that Na⁺ ions are easier than K⁺ ions to intercalate into the layered δ-MnO₂ due to their smaller ion radius. The incorporation of K⁺ and Na⁺ into δ-MnO₂ shows a great influence on the electrochemical performance of the layered δ-MnO₂. The full cell with (K, Na)-co-incorporated δ-MnO₂ (K : Na : Mn = 0.15 : 0.26 : 1) hierarchical nanospheres as the cathode and NaTi₂(PO₄)₃ as the anode exhibits a specific capacity of 74.6 mA h g^-1 at 150 mA g^-1, and the capacity remains at ∼62% at a high current density of 600 mA g^-1. The electrochemical cycling does not induce observable structural degradation even after 200 cycles. K-incorporated and (K, Na)-co-incorporated δ-MnO₂ electrodes have superior capacity and rate capability, which can be ascribed to their hierarchical structure and adequate crystallinity.